Nitrogen polymer compositions having thermally reversible cross-links resulting from treatment with organometallic compounds

ABSTRACT

Polymer compositions and processes for preparing polymer compositions having thermally reversible cross-links are described which compositions comprise a polymer having nitrogen atom-containing groups pendant from the polymer chain cross-linked by co-ordination bonds of nitrogen atoms of adjacent chains with the metal atom of a metal-containing organic compound in which the metal atom is attached to the organic moiety through sulfur or oxygen.

This invention relates to polymeric compositions more particularly tocross-linked polymeric compositions in which the cross-links arethermally labile.

Polymeric materials can be broadly classified as either thermoplastic orthermosetting according to their behaviour on heating. Thermoplasticresins are those which can be softened or melted by heat and while softor molten can be moulded, cast or extruded under pressure. On coolingbelow their softening point, they become rigid and retain the shape ofthe mould. Thermoplastic castings or mouldings, upon being reheated,again become soft and fusible and can be remoulded.

Thermosetting resins include prepolymers and other precursors which canbe softened by heating and while soft can be moulded or cast under heatand pressure. They change chemically, however, during heating, to becomehard, infusible substances, so that once the resin has set or cured itcannot be softened by heating, or remoulded. The difference in behaviourbetween thermoplastic and thermosetting resins is associated with adifference in molecular structure, the former having essentially linearmolecules with little or no intermolecular interaction, while the latterhave a three-dimensional structure with interchain cross-links.

It will be seen that the manufacture of articles from thermosettingresins has certain unfavourable features compared with the use ofthermoplastic resins; in particular there is a limit to the time duringwhich the resin can be held at an elevated temperature before moulding,and there can be no reworking of scrap mouldings. On the other hand,thermoset resins have in many instances desirable physical propertieswhich cannot be matched by those of thermplastic resins.

The conventional vulcanisation of hydrocarbon rubbers, for examplenatural rubber or styrene-butadiene rubber, in which the rubber isheated with sulphur, is, on a broad view, an irreversible reactionanalogous to the curing of a thermosetting resin. The limitation on theduration of holding the mixture at an elevated temperature beforemoulding is encountered in the phenomenon of scorch or prevulcanisation,so that close control of mixing times and temperatures and of theforming processes prior to vulcanisation is essential if high scraprates are to be avoided. Forming operations such as injection mouldingcan, under appropriate conditions, be used in the production ofvulcanised rubber articles. Unlike the injection moulding ofthermoplastics, however, where a cooled mould can be used and thecasting removed very shortly after injection, the vulcanisation requiresthe use of a heated mould and residence in the mould until vulcanisationis complete.

Other methods of vulcanising elastomers, for example the use asvulcanising agents of sulphur-donating compounds or organic peroxides inthe vulcanisation of hydrocarbon rubbers, or the use of metal oxides inthe vulcanisation of polychloroprene, involve irreversible cross-linkformation, and the processing limitations consequent thereon.

Polymeric compositions having thermally reversible cross-links, so thatthe compositions would exhibit properties characteristic of thermosetresins or vulcanised rubbers at service temperatures, but thermoplasticproperties, at higher, processing temperatures, with restoration of theservice temperature properties on cooling, would therefore represent avaluable advance in plastics and rubber technology. Such compositionsand a process for their production are provided by the presentinvention.

In one aspect, a composition of the invention is a cross-linkablepolymer composition comprising a polymer having nitrogen atom-containinggroups pendant from the polymer chain, and an organometallic compoundcontaining a metal atom capable of complex formation with at least twoof the said nitrogen atoms.

In a further aspect, a composition of the invention is a cross-linkedpolymer composition comprising a polymer having nitrogen atom-containinggroups pendant from the polymer chain and wherein cross-links exist byco-ordination of nitrogen atoms of adjacent chains with the metal atomof an organometallic compound, the said cross-links being substantiallythe only type of cross-links in the polymer composition.

The process of the invention comprises incorporating an organometalliccompound into a polymer having nitrogen atom-containing groups pendantfrom the polymer chain, the organometallic compound being one containinga metal atom capable of complex formation with at least two of the saidnitrogen atoms whereby the organometallic compound is capable offunctioning as a cross-linking agent, in the substantial absence ofconditions which would produce permanent cross-links in the polymer.

A further aspect of the invention is a method of producing an articlefrom a composition of the invention, which comprises heating thecomposition to a temperature at which it exhibits smooth flow underpressure, filling a mould with the heated composition, allowing thethus-moulded article to cool to a temperature at which it retains itsform on removal from the mould, and removing the cooled article from themould.

This method can be applied, for example, to the injection moulding ofrubber articles, and enables articles having the physical properties ofrubber vulcanisates to be produced much more rapidly than by aconventional vulcanisation process.

The foregoing method includes the situation where the composition thatis heated has already been heated and cooled once or several timespreviously, as in the recovery of scrap mouldings.

The organometallic compound can be incorporated into the polymer by anyconventional method of mixture, for example on a mill or in an internalmixer, at any temperature at which the polymer is in an adequatelyplastic condition. Mixing is continued until the organometallic compoundis uniformly distributed throughout the polymer.

The cross-linked polymeric compositions of the invention have physicalproperties characteristic of such compositions, e.g. of thermoset resinsor of vulcanised rubbers at temperatures encountered under most serviceconditions, i.e. at temperatures in the range of 0° to 100°C. or even upto 150°C. At higher temperatures, however, for example above 160°C, thebehaviour of the compositions is consistent with scission of the crosslinks, but these, and the associated physical properties, are restoredwhen the composition cools. Thus the temperature at which theorganometallic compound is incorporated into the polymer may be suchthat cross links are not formed during mixing, but during cooling aftermixing.

The nitrogen atom-containing group in a polymer that is cross-linkedaccording to the process of the invention can be for example an aminegroup, an amide group, a nitrile group, or a cyclic group in which thenitrogen atom accurs as a hetero-atom and in which it may or may notexhibit amine nitrogen properties. An amine group in which the nitrogenatom is not part of a ring is preferably a primary amine group, moreespecially one in which the nitrogen atom is linked to a non-aromaticcarbon atom. Such a pendant group can be for example an aminomethyl,2-aminoethyl, 2-aminoethylthio or a 4-(aminomethyl)phenyl group.Preferred examples of heterocyclic groups which are also amine groupsare pyridyl and alkylpyridyl groups, especially groups where thenitrogen atom is in the 4-position relative to the position ofattachment of the pyridyl group to the polymer chain. Another example isthe isoquinoline group. The nitrogen atom-containing groups may beintroduced into the polymer by polymerisation or interpolymerisation ofan appropriate nitrogen atom-containing monomer, which may for examplebe 4-vinyl pyridine, 2-methyl-5-vinylpyridine, acrylamide oracrylonitrile, or by modification of an existing polymer.

In a particular embodiment of the invention, the polymer is an elastomerwhich is a copolymer of a diene, for example, 1,3-butadiene or isoprene,and a nitrogen atom-containing monomer, for example 4-vinylpyridine, oran interpolymer of the diene, a nitrogen atom-containing monomer and atleast one other ethylenically unsaturated monomer. In such elastomersthe additional monomer is preferably styrene or a substituted styrene,but it can, for instance, be an olefin, for example isobutylene, or anester, for example methyl methacrylate.

In the elastomer, the diene is generally the major component, andtypically may represent from 50 to 95%, for example from 55 to 85%, byweight of the elastomer. The nitrogen-containing monomer may constitutefrom 1 to 30% by weight of the elastomer, the preferred amounts beingfrom 2 to 10% by weight, and more preferred from 2 to 7% by weight ofthe elastomer. The additional monomer or monomers may typically bepresent in an amount of from 20 to 30% by weight.

Other polymers to which the process of the invention can be appliedinclude copolymers of nitrogen atom-containing monomers (as exemplifiedabove) with mono-olefins, for instance ethylene, propylene or isobutene,with vinyl aromatic compounds, for instance styrene and substitutedstyrenes, with vinyl chloride, or with mixtures of such monomers witheach other or with other polymerisable monomers.

The organometallic compound that is incorporated into the polymer in theprocess of the invention is preferably one in which the metal isselected from the complex-forming metals of the First Long Period of thePeriodic Table according to Mendeleef, for example, zinc, vanadium,chromium, manganese, iron, and nickel. Particularly good results areobtained using compounds of nickel. Compounds of the complex-formingmetals of the Second and Third Long Periods may also be used, however,for example compounds of cadmium, tin, rhodium or iridium.

The organometallic compound may for example be a carboxylate,thiocarboxylate, alkoxide, or mercaptide of the metal, and is preferablyone in which the metal is already partially complexed, i.e. there ispresent at least one group which is bound to the metal atom by aco-ordinate bond. The organometallic compound is preferably one that isfree from atoms or groups attacked to the metal atom which correspond tothe anions of strong inorganic acids, for example chloride or sulphate.

The complexes of zinc with two molecules of 2-mercaptobenzothiazole(referred to hereinafter as Zn(MBT)₂), and of nickel with two moleculesof 2-mercaptobenzothiazole or two molecules of 2-mercaptobenzoxazole(referred to hereinafter as Ni(MBT)₂ and Ni(MBO)₂ respectively) are veryeffective cross-linking agents for polymers containing pyridyl groups.Nickel acetyl acetonate and nickel dialkyldithiocarbamates are furtherexamples of effective cross-linking agents for such polymers.

The amount of the organometallic compound used will depend on theproperties required in the cross-linked polymer, but is generally withinthe range 0.5 to 10 parts by weight per 100 parts by weight of thepolymer. More usually the quantity is from 1.5 to 7.5, for example from2 to 6, parts by weight per 100 parts by weight of the polymer.

The compositions of the invention can contain additives customarilyincluded in conventional polymer compositions and substantially inertwith respect to the cross-linking process, for example fillers such ascarbon black and silica, antidegradants (i.e. antioxidants orantiozonants), and processing aids. Oil-extended synthetic elastomerscan be used.

Conditions which would produce permanent cross-links in the polymer, andwhich must be avoided in the production of the compositions of theinvention, include exposing the compositions to radiation of a typecapable of generating free radicals at points on the polymer chain, andthe presence in the polymer of various substances known to be effectiveas chemical cross-linking agents. These include organic peroxides,quinones, their precursors such as certain methylol bisphenols, andanalogous substances such as dioximes, sulphur, and sulphur-containingvulcanising agents such as thiuram di- and polysulphides and amine di-and polysulphides.

The method of producing an article from a composition of the inventionrequires the composition to be heated to a temperature at which itexhibits smooth flow under pressure. The temperature at which thiscondition is met will vary according to the nature of the polymer. Inthe case of compositions where the polymer is an elastomericinterpolymer of a diene, 4-vinylpyridine and styrene, good flowproperties are exhibited at temperatures in the range 180°-220°C.

The invention is illustrated by the following Examples.

EXAMPLE 1

An emulsion polymer was prepared using the formulation given below whichwas sealed into a mineral water bottle and tumbled in a water bath at50°C. during 6 hr.

    ______________________________________                                        Water                125.00    g.                                             Potassium sulphate   0.77      g.                                             Sodium bicarbonate   1.08      g.                                             Sodium lauryl sulphate                                                        40% paste            6.90      g.                                             Potassium persulphate                                                                              0.39      g.                                             t-dodecyl mercaptan  0.10      ml.                                            4-vinyl pyridine     3.00      g.                                             styrene              12.00     g.                                             butadiene            35.00     g.                                             ______________________________________                                    

At the end of the polymerisation, evaporation of a sample in thepresence of hydroquinone showed that 82.5% conversion was obtained. Theremaining latex was cooled, short-stopped with hydroquinone andcoagulated by adding the latex to 500 ml. of hot methyl alcohol. Thecoagulum was collected, broken into small pieces and stirred with freshhot methyl alcohol (500 ml.). The polymer was further washed with fourchanges of water at 60°C. then dried at 1.0 mm. overnight. The polymerwas found to contain 0.7% N (Theory 0.8%).

Using this polymer, the mixes listed in the table below were made on arubber mill heated to 50°C. and samples of the mix heated to 180°C. in amodel TM 100 Monsanto Rheometer during 5 min. then cooled to thetemperatures indicated in the table and the sample held at the statedtemperature during 5 min. before further cooling.

                  Parts by Weight                                                 ______________________________________                                        Rubber  10.0      10.0      10.0    10.0                                      Zn(MBT).sub.2                                                                         0.40      --        --      --                                        Ni(MBT).sub.2                                                                         --        0.39      --      --                                        Ni(MBO).sub.2                                                                         --        --        0.36    --                                        ______________________________________                                    

The weights of metal compounds added are equivalent to 10⁻ ⁴ mole/g. ofrubber.

    ______________________________________                                        Temperature (°C)                                                                    Rheometer torque (foot lb.)                                      ______________________________________                                                180°C                                                                           27       24.5   27     24                                            160°C                                                                           30.5     27.5   32.5   26                                            140°C                                                                           33       31     38     30                                            120°C                                                                           37       36     43.5   32                                            100°C                                                                           41       40     49.5   35                                    reheat to                                                                             180°C                                                                           28       25     30     26                                    ______________________________________                                    

The higher values of the torque obtained for the rubber containing theadditives in comparison with the blank show that the rubber containssignificant concentrations of cross-links at 160°C but that at 180°Cthese are broken to give a rubber with a torque very similar to thatobtained for the blank.

EXAMPLE 2

A polymer was prepared under the same conditions as that in Example 1excepting that the polymerisation time was 5 hr. and the conversion 65%.It had a nitrogen content of 0.8% (theory 0.8%).

The mixes given in the following table were prepared using this polymerin which the concentration of Ni(MBO)₂ was varied.

                                      Part by Weight                              __________________________________________________________________________    Rubber      10    10   10     10                                              Ni(MBO).sub.2                                                                             0.18  0.36 0.54   0.00                                            moles/g.                                                                      of Ni(MBO).sub.2                                                                          5 × 10.sup..sup.-5                                                            10.sup..sup.- 4                                                                    1.5 × 10.sup..sup.-4                             Temperature                                                                           (°C)                                                                       Rheometer torque (foot lb.)                                               180 22    24   26     19                                                      160 25    28   32     21                                                      140 29    33   39     23                                                      120 34    39   46     26                                                      100 35    41   46     26                                              reheat at                                                                             180 22.5  25   28     20                                              return to                                                                             100 32    39   43     40                                              __________________________________________________________________________

The differences in the Rheometer torque suggest that increases inconcentration of Ni(MBO)₂ result in higher cross-link densities in thepolymer. The small differences in torque at 100°C obtained after firstcooling from 180°C and then recycling to 180°C and cooling again to100°C show that the thermal lability of the cross-links is reversibleand that the process can be repeated without any significant loss ofcross-links.

EXAMPLE 3

A 4-vinyl pyridine-butadiene copolymer was made by the method given inExample 1 but using 3.0 g. of 4-vinyl pyridine and 47.0 g. of butadiene.The conversion after 5 hr. was found to be 55% and the polymer contained0.7% N.

The torque values obtained on a mix of this rubber with Ni(MBO)₂ incomparison with a blank are shown in the following table.

                  Parts by Weight                                                 ______________________________________                                        Rubber        10.0         10.0                                               Ni(MBO).sub.2 0.36         --                                                 Temperature                                                                             (°C)                                                                           Rheometer torque (foot lb.)                                           180     31           28                                                       160     35.5         30                                                       140     40           32                                                       120     45           35                                                       100     46           37                                             reheat to 180     33           --                                             return to 100     45           --                                             ______________________________________                                    

EXAMPLE 4

A polymer was prepared by the method given in Example 1 excepting thatthe polymerisation was carried out during 5 hr. to a conversion of 65%and 1 phr of an antioxidant (polymeric2,2,4-trimethyltetrahydroquinoline) as a 33% aqueous dispersion wasadded to the latex. The latex was coagulated by freezing in anacetone/carbon dioxide bath and the thawed mixture was extracted tentimes with water at 60°C (600 ml. each time) to extract the emulsifyingagent and inorganic salts.

The crumb was dried, and milled with 0.36 grams of Ni(MBO)₂ per 10 gramsof rubber. A sample of the mixture thus obtained was twice heated to180°C and allowed to cool, and was then cut into small pieces which wereput into a one inch diameter mould heated to 180°C and compressed with acylindrical ram at a pressure of 500 lb. per sq. in. to give a wellmoulded cylinder of rubber one inch in diameter and half an inch thick.This illustrates that vulcanisates cross-linked by Ni(MBO)₂ co-ordinatedwith rubber-bound pyridine groups can be remoulded at elevatedtemperatures.

EXAMPLE 5

The Example shows the use of other nickel compounds as cross-linkingagents in the vulcanisation of a 4-vinylpyridine/styrene/butadieneinterpolymer. A polymer was prepared by the method given in Example 1with a polymerisation time of 5 hr. (when the conversion was 65%), andthe latex was coagulated using methyl alcohol. 2 Phr of antioxidant(polymeric 2,2,4-trimethyltetrahydroquinoline) was swollen into therubber from an acetone solution, and the rubber was dried under reducedpressure. The mixes given in the table below were made up on a 6 inchrubber mill heated to 40°C.

                  Parts by Weight                                                 ______________________________________                                        Rubber      20.0     20.0     20.0   20.0                                     Nickel II Acetyl                                                              Acetonate   0.51     --       --     --                                       Nickel II                                                                     Hexanoate   --       0.58     --     --                                       NiMBO       --       --       0.72   --                                       Temperature (°C)                                                                   Rheometer Torque (foot lb.)                                       180         17       18       20     17                                       160         20       25       24     18.5                                     140         23.5     30       28.5   21                                       120         28       34       34.5   24                                       100         28       35       25     21                                       ______________________________________                                    

EXAMPLE 6

In another test, cross-link formation in compositions of the inventionwas demonstrated from their behaviour on compression.

Various organometallic compounds, as set out in the table below, weremixed into samples of the 4-vinylpyridine/styrene/butadiene interpolymerof Example 5, in an amount of 10⁻ ⁴ mole of organometallic compound pergram of interpolymer.

Test samples were prepared by placing 8.3 grams of the mixture, cut intosmall pieces, into a cylindrical mould which was then heated to 180°C.The mould was closed with a piston to which a pressure of 400 p.s.i. wasapplied and maintained for 3 minutes. After cooling to 100°C or below,the cylindrical test piece was removed from the mould and allowed torest for 24 hours before being tested.

The tests were carried out with the sample at 20°±2°C using a standardtensile testing machine fitted with a compression cage, and at acompression rate of 5 mm. per minute. The sample was examined fordimensional recovery 30 minutes after release of the maximum compressiveforce.

The results are shown below.

    ______________________________________                                                   Thickness(mm) Force (kg) for Com-                                  Organo-                  pression                                             metallic            30 min.  25%   40%   50%                                  Compound   Before   after                                                                Com-     Com-                                                                 pression pression                                                  ______________________________________                                        None       8.3      --       26.5  53     85                                  Nickel                                                                        Hexanoate  8.54     8.53     33.5  72.5  100                                  Nickel Acetyl-                                                                acetonate  8.59     8.59     37.5  79.0  100                                  Zn(MBT).sub.2                                                                            8.51     8.46     30.5  62.5  100                                  Ni(MBT).sub.2                                                                            8.30     8.26     33.0  66.6  100                                  Nickel dibutyl-                                                               dithiocarbamate                                                                          8.60     8.58     33.0  66.6  100                                  ______________________________________                                    

All the samples containing organometallic compounds showed greaterresistance to compression than the blank, from which cross-linkformation by the organometallic compound can be deduced. The samplesshowed good dimensional recovery after compression.

EXAMPLE 7

An elastomeric interpolymer was prepared by the method of Example 1 froma monomeric mixture of 6% by weight of 4-vinylpyridine, 24% by weight ofstyrene and 70% by weight of butadiene. The elastomer had a Mooneyviscosity (ML-100) of 40.

Nickel acetyl acetonate was milled into two samples of the elastomer inamounts corresponding to 5 × 10⁻ ⁵ and 10⁻ ⁴ mole of nickel acetylacetonate per gram of elastomer respectively (1.3 grams and 2.6 grams ofnickel acetyl acetonate per 100 grams of elastomer respectively) on a 9inch rubber mill at 40°-50°C. The mixtures were moulded using a transfermould consisting of a heated reservoir and plunger connected through asmall orifice to a cavity mould 15 cm square and 2 mm in depth. Formoulding, the elastomer was placed in the reservoir, the reservoir andmould were heated to 190°-200°C. and the elastomer was transferred tothe mould by operating the plunger.

Measurements of the physical properties of the sheets of elastomer thusformed were carried out on test pieces cut from the sheets with aBritish Standard Type 2 dumb-bell cutter. Results are shown in thefollowing table in which A is the mixture containing 5 × 10⁻ ⁵ and B themixture containing 10⁻ ⁴ mole of nickel acetylacetonate per gram ofelastomer.

    ______________________________________                                                           A         B                                                Tensile strength (Kg/cm.sup.2)                                                                   24        34                                               Elongation at break (%)                                                                          200       210                                              Modulus 100%       13        14                                                   200%           24        33                                               ______________________________________                                    

The cross-link agents of this invention are metal containing organiccompounds in which the metal atom is attached to the organic moietythrough sulfur or oxygen. Compounds containing metals of atomic numbersof 22 to 30 are preferred. The sulfur or oxygen atoms are connected to acarbon atom of a heterocyclic radical, for example, as in an azole whichis a type of a mercaptide; to a carbonyl or thiocarbonyl, for example,as in a carboxylate, thiocarboxylate or dithiocarbamate; to an aliphaticcarbon atom, for example, as in an alkoxide or alkyl mercaptide; or to aphosphorous atom, for example, as in a phosphorodithioate.

Examples of carboxylates are the metal salts of alkanoic and alkenoicacids having, for example, up to 20 carbon atoms per molecule, forexample acetic, propionic, butyric, hexanoic, octanoic, lauric,myristic, palmitic, stearic, acrylic, crotonic and oleic acid; metalsalts of aromatic monocarboxylic acids for instance benzoic, toluic,salicylic and naphthoic acids; metal salts of polycarboxylic acids, forexample oxalic, succinic, adipic, maleic, fumaric, citric, phthalic andpyromellitic acids. Examples of thiocarboxylates are thioacetates,dithioacetates, thioglycollates, dithiobenzoates, dithiosalicylates,dithio-oxalates, dithiocarbamates, N,N-dialkyldithiocarbamates andN-phenyl-N-alkyldithiocarbamates where the N-alkyl groups contain up to6 carbon atoms, for example N,N-diethyldithiocarbamates,N,N-dibutyldithiocarbamates and N-ethyl-N-phenyldithiocarbamates.

An alkoxide may typically be one derived from an alcohol having from 1to 12 carbon atoms per molecule. Also included are phenates, includingnuclear-substituted phenates such as chlorophenates and alkylphenates.

The mercaptides may for example be derived from alkyl mercaptans,aralkyl mercaptans, thiophenols or from heterocyclic mercapto compounds.The alkyl mercaptides are generally C₁₋₁₂ mercaptides, for example ethylor butyl mercaptides. Benzyl mercaptan is an example of an aralkylmercaptan. The thiophenols include nuclear-substituted thiophenols, forexample chlorothiophenols and alkylthiophenols. Also included aremercaptans and thiophenols containing more than one --SH group, forexample ethylene dithioglycol and dithiocatechol. Examples ofheterocyclic mercaptans are 2-mercaptothiazole, 2-mercaptobenzothiazoleand 2-mercaptobenzimidazole.

Also useful in the present invention are metal salts ofphosphorodithioic acids of the formula (RO).sub. 2 P(=S)SH where each Rrepresents a hydrocarbon or substituted hydrocarbon group, for instancean alkyl, cycloalkyl, aryl or aralkyl group. Examples are described inU.S. Pat. Nos. 3,308,103 and 3,496,152 the disclosures of which areincorporated herein by reference. Zinc O,O dialkyl phosphorodithioatesof 1-20 carbon atoms are illustrative of this type of compound.

Suitable compounds include zinc butyrate, zinc hexanoate, zinc stearate,zinc thiophenate, manganese acetate, ferric ethylate, ironN,N-diethyldithiocarbamate, nickel butyrate, nickel hexanoate, nickeloctanoate, nickel pentachlorothiophenate, nickel dibutyldithiocarbamate,nickel dibutylphosphorodithioate, cadmium diisopropylphosphorodithioate, tin cyclohexyl methylphosphorodithioate.

Examples of reagents which form complexes with the metals of the FirstLong Period of the Periodic Table include diamines such as ethylenediamine, alpha,beta-propylene diamine, alpha,alpha prime-dipyridyl,o-phenanthroline; derivatives of alpha-diketones such as the mono anddioximes and the mono and dihydrazones of diacetyl and benzil,beta-diketones for instance acetylacetone and benzoylacetone and theirderivatives such as the mono and dioximes. Other reagents includesalicylaldoxime, dicyanodiamidine, nitrosophenylhydroxylamine,8-hydroxyquinoline, glycine, alpha,beta,gamma-triaminopropane,ethylenediamine tetracetic acid, and various organo-substitutedphosphines and arsines, for example triphenylphosphine andtriphenylarsine. Certain carboxylates, thiocarboxylates and mercaptidesare more properly regarded as complexes rather than simple salts, as aremetal derivatives of for example catechol.

The complexes may be ionic or non-ionic. Where the complex is cationic,the anion is preferably organic, for example the anion of a carboxylicacid or of an organic sulphonic acid. Specific examples of complexesthat can be used in the process of the invention are the complex of zincwith two molecules of 2-mercaptobenzothiazole (referred to hereinafteras Zn(MBT)₂), complexes of vanadium with acetylacetone and withbenzoylacetone, vanadyl complexes of the formula M₂ [VO(OC₆ H₄ COO)₂ ].3₂ O and M₂ [VO(C₆ H₄ O₂)₂ ]C₆ H₆ O₂.H₂ O where M represents ammonium oran alkali metal, the coordinating groups in such vanadyl complexes beingderived respectively from salicyclic acid and catechol, complexes ofchromium with ethylene diamine, alpha,alpha prime dipyridyl and withacetylacetone, and oxalate complexes of the formula M₃ [Cr(C₂ O₄)₃ ]where M is ammonium or an alkali metal, complexes of manganese withacetylacetone and with benzoylacetone and ionic complexes of the formulaM₃ [MnA₃ ] and M[MnA₂ (OH.sub. 2)₂ ] where A is the divalent anion ofoxalic or malonic acid, the complexes of iron withnitrosophenylhydroxylamine and iron complexes containing bothbeta-diketonic and amine coordinating groups, for instance (AcAc)₂FePy₂, where AcAc represents acetylacetone and Py represents pyridine,the complexes of nickel with two molecules of 2-mercaptobenzoxazole andwith two molecules of 2-mercaptobenzothiazole (referred to hereinafteras Ni(MBO)₂ and Ni(MBT)₂ respectively), complexes of the formula Ni(en)₃X₂, Ni(dipy)₃ X₂ and [Ni(en)₂ (OH₂)₂ ]X₂ where en represents ethylenediamine, dipy represents alpha, alpha prime-dipyridyl, and X representsacetate or X₂ represents oxalate, the non-ionic nickel complexes of theformula NiG₂ where G represents dimethylglyoxime, methyl ethyl glyoxime,methyl propyl glyoxime or methyl benzyl glyoxime, the complex of nickelwith salicylaldoxime, the complex of nickel with dicyanodiamidine, andthe complexes of nickel with acetylacetone and with benzoylacetone.

Other examples are zinc 2-benzamidothiphenate, zinc thiophenate, zincethyl mercaptide, cupric dimethyldithiocarbamate, cupric O,O diisopropylphosphorodithioate, zinc diisopropyldithiocarbamate, zinc O,Odiisopropylphosphorodithioate zinc di-n-butyldithiocarbamate, zincdi-n-butylphosphorodithioate, cobalt 2-mercaptobenzothiazole, nickel2-mercaptobenzothiazole, zinc 2-mercaptothiazole, nickel2-mercaptothiazole, and zinc 2-mercaptobenzimdazole.

Although the invention has been illustrated by typical examples, it isnot limited thereto. Changes and modifications of the examples of theinvention herein chosen for purposes of disclosure can be made which donot constitute departure from the spirit and scope of the invention.

What we claim is:
 1. A polymer composition comprising an elastomericpolymer having nitrogen atom-containing groups pendant from the polymerchain whch polymer is a copolymer of a conjugated diene and a nitrogenatom-containing monomer or an interpolymer of a conjugated diene, anitrogen atom-containing monomer and at least one other ethylenicallyunsaturated monomer, and a cross-link agent consisting essentially of anorganometallic compound selected from the group consisting of metalcarboxylate, a metal thiocarboxylate, metal alkoxide, mercaptide, metaldithiocarbamate and metal phosphorodithioate containing a metal atomwhich forms thermally reversible cross-links with the nitrogen atoms ofat least two of the said pendant groups.
 2. The composition of claim 1in which the amount of cross-link agent is within the range of 0.5 to 10parts by weight per 100 parts by weight polymer and in which thenitrogen atom-containing monomer is 4-vinylpyridine.
 3. The compositionof claim 2 in which the metal is zinc or nickel.
 4. The composition ofclaim 3 in which the organic compound is one in which the metal isalready partially complexed.
 5. The composition of claim 4 in which theorganic compound is a complex of zinc with two molecules of2-mercaptobenzothiazole, a complex of nickel with two molecules of2-mercaptobenzothiazole, or a complex of nickel with two molecules of2-mercaptobenzoxazole.
 6. The composition of claim 4 in which theorganic compound is nickel acetylacetonate.
 7. The composition of claim3 in which the organic compound is a nickel carboxylate or a nickeldialkyldithiocarbamate.
 8. A cross-linked polymer composition comprisingan elastomeric polymer having nitrogen atom-containing groups pendantfrom the polymer chain which polymer is a copolymer of a conjugateddiene and a nitrogen atom-containing monomer or an interpolymer of aconjugated diene, a nitrogen atom-containing monomer and at least oneother ethylenically unsaturated monomer in which the cross-links consistessentially of thermally reversible cross-links formed by co-ordinationof the nitrogen atoms of pendant groups of adjacent chains with themetal atom of a metal-containing organic compound selected from thegroup consisting of metal carboxylate, metal thiocarboxylate, metalalkoxide, mercaptide, metal dithiocarbamate and metalphosphorodithioate.
 9. The composition of claim 8 in which the amount ofcross-link agent is within the range of 0.5 to 10 parts by weight per100 parts by weight polymer and in which the nitrogen atom-containingmonomer is 4-vinylpyridine.
 10. The composition of claim 9 in which themetal is zinc or nickel.
 11. The composition of claim 10 in which theorganic compound is one in which the metal is already partiallycomplexed.
 12. The composition of claim 11 in which the organic compoundis a complex of zinc with two molecules of 2-mercaptobenzothiazole, acomplex of nickel with two molecules of 2-mercaptobenzothiazole, or acomplex of nickel with two molecules of 2-mercaptobenzoxazole.
 13. Thecomposition of claim 11 in which the organometallic compound is nickelacetylacetonate.
 14. The composition of claim 10 in which the organiccompound is a nickel carboxylate or a nickel dialkyldithiocarbamate.